In electrophotography, generally, a surface of a latent image bearing member is charged using corona discharge etc. followed by exposure using laser etc. to form an electrostatic latent image, which is then developed to form a toner image, and the formed toner image is further transferred on a recording medium to obtain an image with high quality. The toner used for the electrophotography is typically one produced by mixing a binder resin such as thermoplastic resin with a colorant, a charge control agent, a release agent, a magnetic material, and the like, then the mixture is further mixed and kneaded, pulverized, and classified to form toner particles with an average particle diameter of from 5 μm to 10 μm. In order to provide flowability to the toner, to perform charge control of the toner, and/or to improve cleaning ability of the toner, silica and/or inorganic fine particles such as those of titanium oxide are externally added to the toner.
In recent years, such a toner is required to have a smaller particle diameter so as to improve resolution from the viewpoint of higher image quality of formed images. It is effective for producing images with high quality that average particle diameter of the toner is reduced to about 5 μm and also particle size distribution of the toner is made sharp.
For this reason, a method of aggregating fine particles has been attracting attention, in which fine particles of components such as a binder resin, a colorant, and a release agent in the toner are aggregated and then the formed aggregates are heated and coalesced to produce the toner, since shape of the toner and dispersibility of components such as a colorant and a release agent in the toner may be easily controlled in addition to particle size distribution of toner particles. In relation to the method of aggregating fine particles, a toner production method has been proposed, in which a mixture of a binder resin, a colorant, a release agent, and an aqueous medium is added with a dispersant such as methylcellulose and hydroxyethylcellulose to prepare a dispersion, and the components in the dispersion are aggregated, and then the formed aggregates of fine particles are coalesced to obtain a toner.
It has been confirmed that a toner with a small particle diameter can be obtained by the above-mentioned method using a certain dispersant. However, in this method, aggregation of pigment fine particles themselves tends to occur when using a pigment as a colorant in the step of aggregating fine particles of the components such as a binder resin, a colorant, and a release agent. Therefore, this method tends to cause compositional variation in the aggregates consisting of fine particles of the binder resin, fine particles of the colorant, and fine particles of the release agent. The occurrence of the compositional variation may possibly lead to a broad distribution of charge amount of the obtained toner. In such a case, image density of formed images may become lower than a desired value when printing for a long time.
In order to solve the problem of aggregation of pigment fine particles, a method may be envisaged in which a large amount of an auxiliary dispersant is added to enhance dispersibility of the pigment fine particles when preparing the pigment fine particles. However, in this case, aggregation of the fine particles may be partially disturbed in the step of aggregating the fine particles of components such as a binder resin, a colorant, and a release agent, and thus the particle size distribution of the obtained toner may become broad. In this case, charge amount distribution of the toner is also broad since surface areas of toner particles are different depending on their particle diameters.